I have had this on my mind for awhile now and was wondering if anyone, particularly DL pilots, would have the answer.

A while back, I was going from ATL to SAT on an M80 and as we were taxiing out to 26L, there were several aircraft ahead of us. I generally like to sit near the rear by the engine to hear the sound, plus it has a hypnotic buzz at cruise. Anyway, I looked out the window and noticed that the right engine was not running and we were taxiing with only the left engine.

Once we started getting closer to 26L, I looked out the window again back at the engine and I heard it start and saw the fan start.

The aircraft was fairly full and we had to wait quite awhile to take off, did the pilot do this to save fuel or what. I had heard that it's harder to steer an aircraft with only one engine running.

I had heard that it's harder to steer an aircraft with only one engine running.

It certainly is harder due to the asymetrical thrust, but at low speeds that wouldn't really be an issue. I think the second engine wasn't started until later just to save a bit of fuel, or maybe to reduce noise for whatever reason...

Pretty common practice...the Operating Manual states "Single engine taxi unless operational needs dictate otherwise"...so if you're at A concourse, and have to get all the way down to Rw 26L, single engine will save alot of fuel...especially with an airline that's trying to slim down gas costs...

Ive had it happen on the -80s too, although most recently on an AAMD-83 in STL.

I was thinking about the asymmetrical thrust of one engine taxying and wouldnt it be less significant than an airliner with wing mounted engines because the -80 engines are nearer the center of the fuselage?

Actually, it's most jet airliners that I can think of...except the 747 lol. Some airports (LGA) you cannot use single engine, as using only one would make ALOT of jetblast to make that breakway power...

Aircraft use a decent amount of fuel while holding short on the ground, and saving any little bit will add up to millions across the fleet....also, judicious use of the APU is important...saves money on gas and cycles on the powerplants...

Was thinking about the asymmetrical thrust of one engine taxying and wouldn't it be less significant than an airliner with wing mounted engines because the -80 engines are nearer the center of the fuselage?

The asymmetrical thrust of a one-engine-running aircraft with rear-mounted engines is certainly less significant than that of an aircraft with wing-mounted engines (after all, torque = force X lever arm ). It is definitely still there, though...I'll let you find the appropriate data and calculate the difference...

And if you want easy evidence of what QantasA332 just said about asymmetrical thrust, compare the size of an MD-80 tail with the size of a 737 tail. Both need to keep the plane from yawing if an engine fails, but there is much less torque with the engines closer to the centerline.

"There are no stupid questions, but there are a lot of inquisitive idiots."

Fuel economy is the only reason to do this...
Jet engines really do not require "warm-up", except in some extreme cold conditions (oil)...
In a 747 we start the "remaining engines" at least 3 minutes before T/O.
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With the 747, in my PanAm days at JFK, FRA or LHR, taxi on 3 or even 2 engines was frequent. I remember being number... something like 40 or 50 for takeoff. Out of JFK, late evening departures for Europe. Sometimes the taxi time did exceed 1 hour. Once at JFK, I also had to return to gate to refuel.
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Do not believe that a tail mounted engine does not have much asymmetrical trust. Even on a MD-80, loss of engine at low speed on the takeoff roll, it is impossible to stay on the runway unless the remaining engine is immediately retarded. There is a VmcG for these airplanes... even if it is lower than airplanes with wing mounted engines, but there is one...
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Happy contrails
(s) Skipper

If you are doing a single engine taxi it's always a good idea to have the APU up and running. If the IDG decides to quit on you your APU gennerator will take over. Much better option than having the airplane go dark and emergency power kicking in.

In the bizjets we fly we do quite a bit of single-engine taxiing. However, most of the time it's after we land, not usually done before takeoff. The reason isn't for fuel savings, but rather to reduce brake wear and keep the taxi speed down. Even at idle thrust, we can really build up quite a head of steam if we're not careful. We have the option of using the TRs, but even those have limitations. On extended taxis, it's just simpler to shut down the RH engine (after the 3 minute cool down period) and bring it in on one engine.

Do not believe that a tail mounted engine does not have much asymmetrical trust. Even on a MD-80, loss of engine at low speed on the takeoff roll, it is impossible to stay on the runway unless the remaining engine is immediately retarded.

Yes asymmetrical thrust of a rear-engine aircraft is certainly still existent (and very forceful, I'm sure), but for the same amount of thrust as a wing-mounted jet, the yawing moment is considerably less, for the reason I stated in my previous post...it's certainly significant, but less.

Same happened to me in a 757 at MIA... We had to wait 15 mins. on the taxiway for the gate, which was busy with another 757 that was departing late, and #1 they shut down the plane, waited 5 mins, turn on Engine #1 and taxied to the Gate with #2 turned off... Cool practice to save fuel...
On the other hand we pushed back one month ago from DFW with trust reversers... weird, huh? It was a MD-80 with AA gain so it was veerrryyy noisy...

Never mind the single engine taxi, what about a No engine taxi, using only the APU !!

Down in Maputo several years ago, there was the usual gaggle of Antonov 26s etc.

Working in the hangar one day, one was always aware when one of those things started its APU. When the APU was loaded for engine start (Pneu) the noise (residual thrust) increased to a level louder than a BAC 1-11 at take off. APU located in rear or nacelle.

Then the noise become increasingly louder, as the aircraft taxied passed the hangar with both props stationary but the APU was providing thrust.

I daresay the aircraft was only repositioning, no doubt lightly loaded for parking, but seemed incredibly weird, and absoloutely deafening

...but is it even possible to see the engine turning while taxiing and in flight?

Yes, it certainly is -- engine running = blurred fan blades, engine not running = distinct/seperate fan blades. The only reason the they (the fan blades) aren't blurred in the photo you refered to is that the view is that of a camera, and thus one fraction of a second of time is captured. If you were simply looking out of the window yourself, it woud definitely be blurred...

Yes, it is possible. If you torque your body just right like Bruce Leibowitz did to get the picture you can see the blades of the fan just past the front "guard ". The first time I looked they were not moving, then the second time, I heard the engine start and saw the blades start spinning. Believe me, it's hard to do and you have to get the right seat just in front of the engine and the aircraft facing toward the sun but its possible. I got cramp in my neck trying to see but it was fun

By the way, thanks for posting that pic. I was looking for it the other day when I posted this. Thanks

When I fly FL and we taxi to Runway 3-2 for takeoff, we run on one engine until we get close to the runway (since the taxi time is so long). If we go to Runway 1-4, The Captain will start both engines before taxiing.